There are various techniques available to mount electronic chips and circuits. In the field for embodying flat screens, a distinction is made between COB (chip on board), COF (chip on flex) and COG (chip on glass) techniques.
These mounting techniques are used to mount on the border of flat screens electronic components for controlling the display of image points or pixels of the screen.
For reasons of simplification of the connections and within a wider integration perspective, the COG technique proves to be one most suitable for flat screens.
In the COG technique, which is a mounting of components on a glass structure, again several variants can be distinguished.
One of these variants is the glued COG technique which for the most part consists of a mounting of electronic chips by means of an elastomer. Although relatively inexpensive to implement, this variant poses problems linked to the individual mounting of the chips, the effectiveness of the mounting and the poor contact resistances and the problems of reliability in a humid atmosphere. The replacement of a chip is moreover almost impossible.
Another variant of the COG technique by means of thermocompression, apart from the problems of individual mounting of the chips and mounting efficiency, does pose problems linked to the pressures required to mount the chips, said pressures possibly generating microcracks in the integrated, circuits.
Furthermore, this technique involves the use of thin connection bosses which therefore limit the mechanical resistance of the chips, especially when the materials constituting the chips and that of the connection support have different thermal cubic expansion coefficients.
A further known technique is the COG wire bonding technique.
According to this variant, connection blocks of the component to be connected and blocks of the connection support are connected by means of wires. As each wire needs to be welded to two blocks, this technique does pose effeciency problems for multi-chip systems having a large number of contacts, and for encapsulating chips close to one another. Moreover, the possible replacement of a defective chip proves to be particularly delicate.
Finally, so as to meet the requirement for increasing the number and complexity of the components or integrated circuits, as well as their operating frequency, a fourth variant for embodying mounting on COG glass is the one known as the SCOG (Solderable Chip on Glass) technique. It makes use of the "Flip Chip" technology for remelting bosses made of a meltable material.
The documents (1), (2), (3) and (4) referred to at the end of the description illustrate these various chip mounting variants.
Although the flip-chip technique seems to be particuarly advantageous for mounting chips on flat screens as mentioned above, its use is not merely limited to mounting chips on glass, but may also concern the mounting of a chip in a casing, the mounting of chips on multichip modules or even on various sensors, for example.
An example of the flip-chip technique is shown diagrammatically on FIGS. 1A and 1B.
As shown on FIG. 1A, this technique initially consists of placing balls 2 made of a meltable metal and alloy on a first structure to be connected 4 on electric conductive blocks 6 made of a material able to be wetted by the material of the ball, and surrounded by an area made of a non-wettable electric nonconducting material.
For reasons of simplification, only one ball 2 is shown on FIG. 1. The structure 4 provided with balls 2 is mounted onto the connection support 10, also having conductive blocks 12 made of a wettable material, surrounded by an area 14 of a non-wettable nonconducting material and intended to receive the balls 2.
The structure 4 with the balls and the support 10 are heated to a temperature exceeding the temperature for melting the metal of the balls 2. This heating thus makes it possible to solder the blocks 6 on the blocks 12 by means of the balls, as shown on FIG. 1B.
The support 10 may be a casing, a substrate with integrated circuits, a sensor or a flat screen. Similarly, the structure 4 may be an electronic chip, a cap or another integrated circuit substrate. The balls 2 may moreover be replaced by disks made of a meltable material having the same function and may be disposed prior to soldering on the blocks 6 and also on the blocks 12.
The heating of both the support 10 and the structure 4 may take place either by convection, for example by a flow of hot air or in a through-oven, or by conduction, for example with the aid of a heating plate applied to the support 10, or even by radiation.
All these solutions generally result in a heating of all the elements (connection support 10 and structure 4) to a temperature exceeding the temperature for melting the metal or alloy of the ball.
Now, this temperature may be incompatible with certain members of the elements to be interconnected.
In particular, when the connection support forms part of a flat liquid display screen, the melting temperature of the balls is excessive.
By way of example, if the meltable metal of the balls is indium, the elements need to be heated to a temperature of about 180.degree. C. which is incompatible with the maximum temperature supported by the liquid crystal cell, namely about 100.degree. C.
One object of the invention is to also provide a heating device able to solder the chips or structures to be connected onto the connection supports, yet still being compatible with the maximum temperatures able to be supported by the structures using said supports, such as flat display screens.